This invention relates to improvements in method and apparatus for injection molding of material such, for example, as thermoplastic resinous material.
In the injection molding of thermoplastic material, the material is plasticized prior to injection into a mold by means of a reciprocating screw plasticizing unit. In such a unit, the material to be molded is fed in granular, powder or other form to a plasticizing barrel to be transported by rotation of the screw to the head of the plasticizing barrel. The material is heated and mechanically worked to a plastic state during its movement by the screw through the plasticizing barrel.
The accumulation of material at the cylinder head results in the displacement of the extrusion screw, thereby pushing the screw backwards in the plasticizing barrel. When enough material has been accumulated to fill the mold, a valve in the injection nozzle is opened and the extrusion screw, acting as a ram, is pushed forward by means of an injection piston mounted for reciprocating movement and operatively connected with the screw. This forward movement of the screw under the action of the piston causes the plastic material to be injected from the plasticizing barrel into the mold. Upon completion of the injection stroke, the screw is rotated again so as to replace the injected volume of material and the cycle is then renewed.
It has been found that on rotation of the screw, the interaction between the helix of the screw and the plastic material, which latter has a high viscosity, causes the screw to push back along the plasticizing barrel before the plastic material reaches the head of the plasticizing barrel. To offset this effect, it has been the practice heretofore to restrict the backward movement of the screw by restricting the displacement of hydraulic fluid from the injection cylinder in which an operative piston is connected with the screw. This can be achieved either by including a flow restrictor in the hydraulic return pipe or by causing the displaced fluid to return to a pressure relief valve. There is a disadvantage in the use of the flow restrictor in that the pressure generated in the injection cylinder is dependent upon the plasticizing rate and will therefore vary with changes in the speed of the plasticizing screw. A disadvantage in both systems suggested is that the screw would start to rotate before the back pressure has been produced. This would cause pockets of gas and volatile substances to be present in the volume of material to be injected. Also, with the systems at present in use, the low pressures involved, especially at low screw speeds, result in difficulties of accurately controlling the valves.